Demodulator for frequency modulated signals



Oct. 11, 1949. T. s. CUSTIN 2,484,556

DEMODULATOR FOR FREQUENCY MODULATED SIGNAL$ Filed Nov. 12, 1946 2Sheets-Sheet l Fig.2.

C'L IPPE 19 Inventor": Thomas G. Custin,

by ,d'm

His Attorney.

Oct. 11, 1949. I T. G. CUSTIN 2,484,556

DEMODULATOR FOR FREQUENCY MODULATED SIGNALS Filed Nov. 12, 1946 2Sheets-Sheet 2 Fig. 8. J\

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H is Attorney Patented Oct. 11, 1949 DEMODULA'TOR FOR FREQUENCY.MODULATED SIGNALS Thomas; G. Ciistimsyracuse, N..Y.', assignor toGeneral. Electric Company, a. corporation of:

New York.

Application November 12, 1946, Serial 'No. 709,337.

1 Claim. 1.-

Myinvention relates to. an' improved counter type. demodulator forfrequency or phase modulated signals.

An] objectrof myinventionis to provide. a demodulating deviceiorifrequency or phase modulated signals .which will providewa highoutput signal level;

A- further object is'sto provide. a demodulating deviceuincorporatingmeans for preventing the passage: of; noisei impulses in. the absence ofa received: carrier: of I at least: a predetermined intensity:.

Another object of my invention is to. provide a demodulator'for phase;or: frequency modulated signals: with 1 high. signaleto-noise levelcharacteristics;.

Aniadditional-object is to:eliminate the necessitysf'or high-gainoutputamplifiers in a phase or frequency modulation" receiver and therebyreduce-the distortion andnoise level.

The novel featuresiwhichlbelieve toxbe characteristic of my inventionareset forth with particularity in the'appended claims. My inventionitself-,.however,z.both as. to its organization and method of'operation; together with further objects. and advantages thereof maybest be understood by referenceto' the following description taken inconnection with the accompanying drawings, in which Fig. 1v is-a diagramof demodulator apparatus-embodying; the invention; Fig. 2 is a diagramof a'demodulator incorporating a squelch circuityand Fig. 3is-amexplanatorydiagram illustrative of the operation of the apparatus.

Counter type'demodulators have been known in the. past. wherein a.frequency-modulated carrier wave is successively subjected to clipping,amplification and differentiation to produce a series of .short pulses,which'in turn arerectified and integrated to produce a demodulated audiooutput signal. Theoutput signal ofa demodulator of this. type. however,is of. relatively low intensity and-must beramplified inv high gainvoltage amplifiers which may introduce undesired noise and distortion.

The.- demodulator,. according to my invention, provides a relativelyhigh intensity output signalpermitting the use of amplifiers of low gainresulting in an output signal characterized by thepresence'ofa minimumof noise and distortion;

According. to my, invention, the frequencymodulated carrier wave isclipped, as in the prior art,. and is then applied to one winding of athree-winding. transformer in which differentiationofi the clipped waveoccurs. The differentiation in this transformer provides very 'sharpapulses, as hereinafter described. Associated with:

the transformer is an' electron discharge device sponding initiatingplifier drives itself to complete saturatlon durmg: each pulse andprovides substantially perfect? limitingof each pulse. The; electrondischarge---. device also serves as a rectifier for the square.

pulses and feeds the rectifiedlpulsesrto atlowrpass filter, such as anaudio passs filter; in which in'--- tegration' is. obtained to: produce;an output signal which is determined by the. rate at whichthessquarepulses are applied torthe filter; Thefilterw output therefore is areproduction-.ofthe. original.

modulation of the incoming carrien.

In the circuit showniinFig. 1, antennazl fure nishes frequency, orphase, modulated; carrier signal energyto. a radio. frequency amplifierand clipper circuit 2', wherein the. incoming" signalr is: modified froma sine wave, suchasrthatlabelledi FM carrier in Fig. 3,.toaclippedsinewave as: shown at 3.111 Fig. 1 and labelled Clippedfinz.Fig. 3.. The clippedwave is applied towindingii. 4 of a. three winding.transformer 5, which is. preferably of the powdered iron. coretype. The?positive-going leading edgeof each clippedpulse;

as-it reaches winding 4 induces. apositive potential in the form of asharp difierentiatedpulse:

at the-top of the secondary windingj' of trans? former 5 to drivethe'control electrode of 81805 tron discharge device I sharply positive.There:-

sultant conduction of device It causes increasing; anode current flow:in" the. third. transformer-r windings, the direction'of the windingsbeing? so chosen-that the increase in anodexcurrentjlnduces an even morepositive potential at thecontrol electrode end of. windingli.This-feedback action quickly drives device-.1 til anode current? Thedevice remains. saturated: fort: a predeterminedtimeduring which amaximum" saturation;

voltage drop occurs across load resistor 9iproviding, a shaped,amplified and rectified. pulse as shown in Fig. 3 for. eachleading-e'dgei ofithez clipped signal. Thev duration of saturation. isestablished by the time.- constants as:- determined by the values ofresistance;inductanceiand distributed capacity of the circuits includingthe transformer windings and,componentsconnectedx; therewith. The.control electrode of device" 1:" may be driven, d'uringthis: time; more;positive:

than the potential required for anode current saturation. When thecontrol electrode potential decays, as the charge accumulated in thedistributed capacity leaks off, the control electrode starts to fallbelow the saturation potential and the anode current starts to decrease.A negative-going potential is thus induced on the control electrode endof winding 6 which serves to hasten cutoif of device 1. Thenegative-going portions of the differentiated pulses, formed by thelagging edges of the clipped carrier waves, do not operate device I. Thedevice operates, therefore, as a rectifier.

The circuit including the transformer and device 1 accordingly operatesin response to the leading edge of each clipped sine wave to provide apositive-going difierentiated pulse, which is generated by induction inwinding 6 by the current in winding l. These positive-goingdifferentiated pulses initate negative-going voltage pulses acrossloadresistor 9 of large constant magnitude due to the self-saturation ofdevice 1.

The negative-going pulses generated by device 1 are; integrated byfilter network l9, which resonates at-a frequency below the frequency ofpulse repetition, such as at less than 250 kilocycles, and theresultantoutput voltage, which may be an audio frequency voltage, represents asummation of the energycontained in the square wave rectifled pulses-The output voltage is, accordingly, an accurate reproduction of thefrequency, or phase, modulation appearing on the received carrier. r

Operating potentials may be supplied for device 7 in a conventionalmanner, anode power being provided from a suitable direct currentptentialsource such as B battery H and control electrode negative biaspotential from a voltage divider. comprising resistors I2 and I3connected across C battery l4. heater for the cathode of device 1 isalso provided though not shown in the drawing.

.The advantages'of demodulation as described above-over the prior artmethods are immediately apparent from Fig. 3, wherein the curves under Aindicate prior art operation, and those under B are exemplary ofoperation utilizing the'zdevic'e-of Fig. 1. In the prior art, as shown,the positive. diiferentiated pulses formed by the leading edges ofclipped input signals waves are rectified'directly. These rectifiedpulses contain onlysm'allquantities of energy, being a little less thanthe energy in the positive differentiated pulses. Not only are therectified pulses inadequately limited for many applications, since therectified pulse energy is related to the energy in the correspondingdifferentiated pulse, but the low intensity of the rectified pulsesrequires high gain amplification to obtain-useable output signalstrength as indicated by the low intensity output 'wave'shown in Fig. 3.

On the. other hand, according to the present invention,v thedifferentiated pulses initiate the generation of almost perfectlylimited square pulses not dependent in energy content on the Aconventionally excited 4 ing 4 of transformer 5. Windin 6 is arranged toprovide positive differentiated pulses to the control electrode ofdischarge device I in response to the leading edges of the clipped sinewaves, and feedback action from winding 8 to winding 6 causes device 1to reach anode current saturation very quickly forjeach positivedifferentiated pulse appearing on winding 6 as explained in connectionwith Fig. 1. Filter network Ill integrates the substantially square wavevoltage appearing across load resistor 9 to provide an output signalcorresponding to the frequency modulation of the carrier received onantenna I.

In addition to the elements described above which are substantially thesame as those of Fig. 1, the device shown in Fig. 2 incorporates asquelch circuit arranged to provide a strong negative cutoff bias to thecontrol electrode of device I in the absence of a received carrier ofgreater than a predetermined minimum intensity. The squelch circuitcomprises a diode discharge device .15 with acathode resistor itconnected to receive they: amplified but unclipped carrier signal fromthe:- radio frequency amplifier section of amplifier and clipper circuit2. When astrong carrier is received, discharge device I5.v conducts,drawing anode current through resistor IT to produce a negativepotential on the-anode. 'A threshold potential for conduction of deviceI5 is established a by selecting the position of the slider of poten-ytiometer I8, which is arranged as a voltage divider across a negativebias potential source- [9 indicated as a C battery.

Electron discharge device .20 is arranged to be conductive when nostrong: carrier: is being received. Power source [9 serves to provideoperat- 3- ing potentials for device 20,- the negative terminal beingconnected to the cathode, the positive (grounded) terminal to'the anode,and a selected Y intermediate potential "from the 'slider" of po-' 1tentiometer I 8 to the control electrode. 5

In the absence of a strong carrier, diode 'device I5 is non-conductiveand there is substan- 3 tially no voltage drop in resistor I]. The con-3 trol electrode of device 20 is thus maintained slider of potentiometerl8, and device 20 conducts to produce a strongly negative potential withrespect to ground on that end of resistor- 2| which is connected to theanode of device at. This negative potential, which may be as'hi'gh asvolts or more, biases the control electrode of device 1 far beyondcutoff preventing the op U eration of the device in response to anynoise impulses which may be received on the antenna I orotherwise. I 7When a carrieris received which is strong enough after rectification bydiode device l5 to produce sufficient voltage drop in resistor 11 toovercome the positive bias on the control elec trodeof device 20, device20 is cut off, conduction through resistor 2! ceases and the con'trolelectrode of device I is made self-biasing through resistors 25 and 22.Condenser 23 funcof device 1.

While I have shown only certainpreferred" embodiments of my invention byway of illustration, many modifications will occur tothose skilled inthe art and I therefore wish'to'have-it understood that .I. intend ,inthe appended claim to cover all .such modificationsas-fall Within thetrue spirit and scope of my invention? What I claim as new and desire tosecure by Letters Patent of the United States is:

A demodulator for frequency modulated clipped carrier wave signalscomprising a transformer, said transformer having an output winding anda feedback winding, an electron diocharge device having a controlelectrode, an anode, a screen electrode and a cathode, a source ofoperating potential having a positive terminal and a negative terminal,the negative terminal being connected to said cathode, one end of saidoutput winding being connected through an impedance to said positiveterminal and the other end of said output Winding being connected toanode, said feedback winding being connected in regenerative polarity atone end to said control electrode and at the other end through a controlelectrode leak resistor to said cathode, means to impress said signalson said control electrode to drive said device to anode currentsaturation in response to impression 0n said control electrode of thepositive-going portion of each cycle of said signal thereby to producecurrent pulses in said anode circuit, a low pass filter connectedbetween said REFERENCES CITED The following references are of record inthe file of this patent:

UNITED STATES PATENTS Number Name Date 2,113,214 Luck Apr. 5, 19382,261,643 Brown Nov. 4, 1941 2,358,297 Bedford Sept. 19, 1944 2,441,957DeRosa May 25, 1948

